Electrical transformers have become more and more powerful over the time, and capable of transforming ever higher voltages, currents, and power. An important limitation of such transformers, especially dry transformers, is their cooling. If cooling is insufficient, some parts of the transformer may overheat. Both heat generation and cooling are generally inhomogenously distributed in the transformer, and some locally overheating portions (hot spots) in the transformer may therefore be present. Such local overheating can reduce the transformer's life-time and reliability drastically.
Therefore, various cooling schemes for cooling the transformers are used. For example, U.S. Pat. No. 2,751,562 describes a dry-type transformer with air cooling. The transformer comprises a baffle member extending from an inner surface of the transformer casing to adjacent the outer periphery of a winding of the transformer, with a space between the outer periphery of the winding member and the adjacent edge of the baffle member.
WO 02082478 describes a liquid cooled and liquid immersed single phase transformer which is enclosed in a tank and uses a pipe system to guide in parallel the cooling liquid through cylindrical chambers surrounding the windings of a first and a second core limb respectively.
GB 691849 describes a liquid cooled transformer which is enclosed in a tank and in which tank the cooling liquid is guided in parallel trough fluid ducts of each of the two coil arrangements from an inlet on the bottom of the side wall of the tank to an outlet on the top of the side wall of the tank. A diaphragm with orifices is provided forcing the cooling liquid through the orifices into the space between the exterior surface of the magnetic core legs and an adjacent cylinder carrying the transformer coils.
U.S. Pat. No. 2,388,565 describes an oil-immersed transformer arranged in a tank with a series cooling circulation which is provided through the inner ducts of a first and a second coil arrangement. The oil circulates from an intake port on the bottom side through various ducts of the first coil arrangement and passes out into a compartment along the outside of the first and second coil arrangement. Then, the oil circulates from the compartment into various ducts of the second coil arrangement and passes out via a separate chamber on the bottom side below the second coil arrangement through an exhaust opening.
U.S. Pat. No. 2,615,075 describes an oil-immersed transformer provided with a cooler in form of a radiator outside the transformer tank. The oil circulates form the bottom side to the top side of the tank through fluid ducts which are formed between the magnetic core and surrounding coils.
Other transformers with cooling means of that class are described for example in DE 909122, DE 1563160, U.S. Pat. No. 2,927,736 and U.S. Pat. No. 2,459,322.
However, with the above transformer the cooling efficiency leaves room to be improved.